AppImage: set a single QtWebEngineProcess path - #5133

Description

Qt reads QTWEBENGINEPROCESS_PATH as a single file path, not as a colon-separated list. Since d9d56d5 (part of #4405) both AppImage recipes set it to two paths joined by :. Qt treats that whole string as one path, can't find a file there, and aborts with Could not find QtWebEngineProcess as soon as the Online Queries dialog is opened.

This PR sets each recipe back to a single path. I used the path where the binary actually is in the 26.2 AppImages (--appimage-extract + find):

• qt5: usr/lib/x86_64-linux-gnu/qt5/libexec/QtWebEngineProcess
• qt6: usr/lib/qt6/libexec/QtWebEngineProcess

Fixes #4899

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

How Has This Been Tested?

I didn't rebuild the AppImages. I tested the released Stellarium-26.2-qt5/qt6-x86_64.AppImage in a GitHub Actions job on my fork (ubuntu-22.04, Xvfb, Mesa llvmpipe). The job extracts each AppImage and runs it twice with a startup script that enables Online Queries and opens its dialog. The first run uses the shipped AppRun.env. The second run changes only the QTWEBENGINEPROCESS_PATH line to the single path from this PR.

The timestamps are high because of software rendering in CI. Both "single path" runs were ended by my job's timeout while still running. They did not exit on their own.

Runs: qt6, qt5. Logs are attached to each run as artifacts.

Test Configuration:

• Operating system: Ubuntu 22.04 (GitHub Actions runner), Xvfb
• Graphics Card: none, Mesa 23.2.1 llvmpipe

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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